treemapstore: make the iterator copy on read after write.

treemapstore: make the iterator copy on read after write.

This may appear to be an odd synchronization scheme, but
there are normally 1 million writes per read, and then at
the end there are no more writes, so this will minimize copies
of the data much more than a traditional copy on write.

Change-Id: I9d78bc2a1de788ca41742e57d0df45d42ac4642a
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/56468
Reviewed-by: Hudson CI
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Tested-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>